With many advantageous properties such as light weight, thinness, low price, low power consumption, and fine joining property with an integrated circuit, the application range of liquid crystal display devices has increased to include such fields as laptop computers, personal digital assistants (PDAs), mobile phones, color television (TV) sets, and the like. A liquid crystal display device is formed of a lower substrate including a light shield film (e.g., black matrix), a color filter, and an ITO pixel electrode; an active circuit portion including a liquid crystal film, a thin film transistor, and a capacitor film; and an upper substrate including an ITO pixel electrode. The lower substrate may be referred to as a color filter array, and the upper substrate may be referred to a TFT array.
The color filter is fabricated by forming red pixels, green pixels, and blue pixels by dispersing pigment particulates in a photosensitive resin composition and the light shield film on a glass substrate. The light shield film blocks light that is not transmitted through a transparent pixel electrode of the substrate in order to prevent the contrast from decreasing due to light transmitted through the thin film transistor, and the red, green, and blue pigment layers have light of specific wavelengths transmitted therethrough among white light to thereby express colors.
In general, a color filter substrate can be fabricated by dyeing, printing, pigment dispersion, electrophoretic deposition (EPD), inkjet printing, and the like. The pigment dispersing method is a method for fabricating a color filter by repeating a series of processes including coating a transparent substrate with a photopolymerizable composition including a coloring agent, exposing the substrate to light through a pattern of a desired shape, and removing unexposed areas with a solvent to thereby thermally cure the resultant structure. The pigment dispersing method can maintain a uniform thickness of a film while improving heat resistance and durability, which are some of the most important characteristics of a color filter, and thus it is widely used for fabrication of a light shield film.
The resist fabricated through the pigment dispersing method is fabricated by using a photosensitive resin composition including two components, a polymer that is a binder resin which functions as a supporter and maintains a uniform thickness, and a photopolymerizable monomer for forming a photoresist pattern by reaction with light during the light exposure. Along with these two components, a pigment dispersion, a polymerization initiator, an epoxy resin, a solvent, and other additives may be also included. Examples of the binder resin used for the pigment dispersion method are a polyimide resin disclosed in Japanese Patent Laid-Open Publication No. Sho 60-237403, a photosensitive resin including an acrylic-based polymer disclosed in Japanese Patent Laid-Open Publication Nos. Hei 1-200353, Hei 4-7373, and Hei 4-91173, a radical polymerization-type photosensitive resin including an acrylate monomer, an organic polymer binder, and a photopolymerization initiator disclosed in Japanese Patent Laid-Open Publication No. Hei 1-152449, and a photosensitive resin including a phenol resin, a cross-linking agent having an N-methylol structure, and a photo acid generator disclosed in Japanese Patent Laid-Open Publication No. Hei 4-163552 and Korean Patent Publication No. 1992-0005780.
Although using a photosensitive polyimide or a phenol-based resin as a binder resin in a pigment dispersion method can provide high heat resistance, there are also drawbacks of low sensitivity and use of an organic solvent for development. Also, a conventional system using an azide compound as a photoresist can have problems of low sensitivity, degraded heat resistance, and an oxygen effect during exposure. Generally, an acrylic-based resin can have excellent heat resistance, shrinkage resistance, and chemical resistance, but tends to have degraded sensitivity, developability, and close contacting (adhesion) properties. Moreover, the sensitivity, developability, and close contacting properties of a light shield film deteriorate more than in other coloring photosensitive resin compositions because the light shield film needs more black pigment to satisfy the required optical density.